In order to obtain large robot reach with low weight and cost, many robot designs have evolved to provide long slender arms. These arms are very suitable for fast motion and entering constricted areas, and so have enjoyed high popularity. However, for precision assembly work they often prove to be inadequate due to excessive backlash and compliance as measured at the end of the robot arm. Also affected by the robots' lack of rigidity are machining operations performed by tools mounted to the ends of the robot arms.
In assembly work, an end effector mounted on the end of the arm is expected to pick up a part, place it precisely, and hold the part without movement while the part is fastened. The fastening process invariably produces forces on the part, and these forces are often of unknown magnitude and direction due to non-controllable conditions of the process. Any backlash and compliance in the robot arm, therefore, contribute to an increase in the production assembly positioning errors.
Where the robot is used in closed loop positioning of the part using machine vision or other feedback measurement, an arrangement is needed to remove, as much as possible, the positioning uncertainty during fastening. Once the part is guided to the correct location, the forces of fastening must be prevented from causing the uncertain movements allowable by the robot arm backlash and compliance.
It is also desirable to use a relatively low precision robot to coarsely position the part and use a high precision end effector to perform the final adjustment. The backlash and compliance of the robot, however, often precludes this approach. For the same reason, machining operations executed by end of arm tooling are limited in performance.